1. Field of the Invention
The present invention relates to a piezoelectric vibrating reed made of a piezoelectric material, such as quartz or lithium tantalate, a piezoelectric vibrator having the piezoelectric vibrating reed, and an oscillator, an electronic device, and a radio-controlled clock each having the piezoelectric vibrator, as well as a manufacturing method of the piezoelectric vibrating reed.
2. Description of the Related Art
Recently, a piezoelectric vibrator utilizing quartz or the like is used in a cellular phone and a portable information terminal as the time source, the timing source of a control signal, a reference signal source, and the like. The piezoelectric vibrator of this type is proposed in a variety of forms, and a piezoelectric vibrator having a turning-fork type piezoelectric vibrating reed is one example. This piezoelectric vibrating reed is a vibrating reed that allows a pair of vibrating arms disposed in parallel to each other to vibrate at a predetermined resonance frequency in a direction to move closer to or away from each other.
As represented by cellular phones, various electronic devices having the piezoelectric vibrator have become smaller in recent years. Accordingly, there is a demand for the piezoelectric vibrating reed of the piezoelectric vibrator to be further reduced in size. Therefore, the piezoelectric vibrating reed is expected to have a structure such that the length of the vibrating arm or base portion thereof is decreased so as to have a shorter total length.
Among this, since the length and width of the vibrating arm are proportional to the frequency of the piezoelectric vibrating reed, it is necessary to decrease the width when the length of the vibrating arm is decreased. However, when the width of the vibrating arm is decreased, there is a problem in that the CI value (crystal impedance) also increases. Therefore, it is difficult to decrease the length of the vibrating arm while maintaining a low CI value.
Therefore, in order to achieve miniaturization of the piezoelectric vibrating reed, it is effective to decrease the length of the base portion. However, since mounting of the piezoelectric vibrating reed is achieved via the base portion, mounting performance thereof will decrease if the length of the base portion is decreased. Therefore, the length of the base portion is typically designed to be as small as possible to an extent that a predetermined mounting performance is secured.
However, it is known that vibration loss (leakage of vibration energy) through the base portion occurs when the piezoelectric vibrating reed is operated. Since this vibration loss leads to an increase of the CI value, it is necessary to suppress the vibration loss as much as possible. In this respect, the vibration loss can be effectively suppressed since the vibration of the vibrating arm can be stabilized by increasing the length of the base portion as much as possible. However, as described above, the length of the base portion is designed to be as small as possible to an extent that predetermined mounting performance is secured from the perspective of miniaturization. Therefore, there is a need to be able to suppress the vibration loss without changing the designed length.
Under such a circumstance, as an effective countermeasure to the vibration loss, as shown in FIG. 19, JP-A-2007-274610 discloses a method of forming a notch portion 202 on either side of a base portion 201.
In this piezoelectric vibrating reed 200, the base portion 201 is divided into an intermediate portion 201a and a mounting portion 201b, which have a different horizontal width, by the presence of the notch portion 202. The mounting portion 201b has a greater horizontal width than the intermediate portion 201a and is a portion which is mainly used when the piezoelectric vibrating reed 200 is mounted. On the other hand, the intermediate portion 201a has a smaller horizontal width than the mounting portion 201b and is a portion which is positioned between the mounting portion 201b and the vibrating arm 203 and to which the base end of the vibrating arm 203 is fixed.
In this way, since by forming the notch portion 202, the mounting portion 201b having a large width can be disposed to be continuous to the intermediate portion 201a, it is possible to effectively suppress the vibration loss. In particular, the length L of the intermediate portion 201a is important, and the vibration loss can be effectively suppressed when the length L is optimum. Therefore, the formation position of the notch portion 202 is designed accurately so that the length L of the intermediate portion 201a becomes optimum.
When the outer shape of the piezoelectric vibrating reed 200 is formed, a protection mask such as a metal mask is generally formed on a quartz wafer, and the protection mask is then patterned so as to comply with the outer shape of the piezoelectric vibrating reed 200. Subsequently, the quartz wafer is etched using the patterned protection mask as a mask, thus obtaining a desired outer shape.
However, there is a case where during the etching, an etching residue 205 as shown in FIGS. 20 and 21 is formed in a step portion between the intermediate portion 201a and the mounting portion 201b. Formation of this etching residue 205 results mainly from the fact that the notch portion 202 is formed in a direction opposite to the crystal direction of the quartz wafer. The shorter the etching time, more of the etching residue 205 remains as shown in FIG. 20. The longer the etching time, less of the etching residue 205 remains as shown in FIG. 21.
When the etching residue 205 is formed, the length of the intermediate portion 201a will change. That is to say, the length (effective value) of the intermediate portion 201a will be L1 for the short etching time as shown in FIG. 20 and L2 for the long etching time as shown in FIG. 21. Particularly, since the amount of the etching residue 205 is likely to vary depending on the etching time, the variation thereof will be great and it is thus difficult to adjust the length of the intermediate portion 201a to a uniform length every time.
Therefore, there is a problem in that the vibration loss also varies and it is unable to stabilize the influence of the vibration loss on the vibration characteristics.